It is a well-known technique to date to use a thermoplastic elastomer, for example, a styrene-isoprene-styrene (SIS) block copolymer, a styrene-butadiene-styrene (SBS) block copolymer or the like as a base polymer for pressure sensitive adhesives and hot-melt type pressure sensitive adhesives. There has also been known a pressure sensitive adhesive composition making combined use of both SIS block copolymer and SBS block copolymer as a base polymer (Japanese Patent Publication No. 42345/1978).
By the way, a hot-melt type pressure sensitive adhesive has a feature that no organic solvent is used in its production stage and coating stage, and it can be handled in a molten state. Thermoplastic elastomers such as SIS block copolymers and SBS block copolymers are rubber-like elastic bodies at room temperature, but have characteristics that their viscosities rapidly decrease as their temperatures rise, and hence they turn fluid state. Therefore, such an elastomer is suitable for a base polymer in the hot-melt type pressure sensitive adhesive.
However, the conventionally-known thermoplastic elastomers of the block copolymer type are poor in heat stability. For example, the SIS type block copolymer undergoes heat deterioration upon its melt processing, and so its melt viscosity rapidly decreases. On the other hand, the SBS type block copolymer undergoes crosslinking upon its melt processing, and so its melt viscosity rapidly increases. Even in a blend system of these copolymers, its melt viscosity rapidly increases due to change with time upon its heating.
Therefore, upon hot-melt processing, hot-melt type pressure sensitive adhesives separately comprising these block copolymers as base polymers require to shorten the storage time of melts before coating and heating time upon processing as much as possible and to strictly control their melt processing temperature. Accordingly, these hot-melt type pressure sensitive adhesives involve a problem that workability upon coating is poor.
On the other hand, there has been proposed a process for producing a heat-stable resinous polymer by first polymerizing styrene in the presence of an organic lithium initiator, next successively adding isoprene and butadiene to polymerize them and then introducing a polyfunctional coupling agent to couple the polymers formed (Japanese Patent Publication No. 46880/1978). However, a polymer obtained by this process is in the form of a resin containing 70-95 wt. % of styrene. Therefore, good pressure-sensitive adhesion properties cannot be achieved if the polymer is used as a base polymer for a pressure sensitive adhesive.